Natural selection can be described in one phrase, "survival of the fittest". When someone says this they don't mean who has the best abs or the biggest biceps, it means whoever is most fit to survive in whatever certain area.
One case that starts natural selection is the struggle for existence. This is when a species is having a hard time in an environment, and needs the most well adapted males and females to reproduce in Order to stay abundant.
Also is variation and adaptation. Natural selection happens here when there is a natural heritable variation.
Evidence of evelution.
There are four separate things that give us reason to believe in evolution. They include:
Biogeography- the study of where animals live now vs. where there ancestors lived
Homologous structures- structures inherited by a common ancestor
Analogous structures- common functions but not structures
Vestigial structures- inherited structures that are useless today.